exports.lookup = function(obj, field) {
    if (!obj) { return null; }
    var chain = field.split(']').join('').split('[');
    for (var i = 0, len = chain.length; i < len; i++) {
      var prop = obj[chain[i]];
      if (typeof(prop) === 'undefined') { return null; }
      if (typeof(prop) !== 'object') { return prop; }
      obj = prop;
    }
    return null;
  };